I. Field of the Invention
The invention relates to analog/radio frequency circuit design. More particularly, the invention relates to an analog/RF switch.
II. Description of the Related Art
A simple switched capacitor sample and hold circuit can be used to convert between an analog continuous time domain and a sampled data domain. FIG. 1 is a conceptual schematic diagram showing a simple switched capacitor sample and hold circuit. Typically the input signal (.nu..sub.in) is a radio frequency (RF) or intermediate frequency (IF) signal which carries a band-limited, modulated signal. The input signal is applied to a switch 20 which opens and closes at a periodic clock frequency. A capacitor 22 is connected between the output of switch 20 and a common ground. The output voltage is generated across the capacitor 22. The capacitor 22 is typically a linear poly-poly or metal-metal capacitor. The output signal (.nu..sub.o) is a sampled data signal. The sampling frequency at which the switch 20 is opened and closed must be higher than twice the modulated bandwidth of the input signal in order to satisfy the Nyquist Theorem. Thus, for a narrow-band signal, the sampling rate can be lower than the carrier frequency as long as it is twice the modulated bandwidth. Using a sampling frequency lower than the carrier frequency of the input signal is referred to as subsampling and is used to downconvert the input signal to a lower frequency.
The spectrum of the output signal contains copies of the input signal centered around multiples of the sampling frequency. For example, the spectral content of the output signal (.function..sub.out) can be expressed as shown in Equation 1. EQU .function..sub.out =n.function..sub.clk.+-..function..sub.in (1)
where:
.function..sub.clk is equal to the sampling frequency; PA1 .function..sub.in is equal to the frequency of the input signal; and PA1 n is equal to 0, 1, 2, 3 . . . . PA1 .nu..sub.in is the voltage level of the input signal; PA1 .nu..sub.out is the voltage level of the output signal; PA1 C is the capacitance value of capacitor; and PA1 R is the on-resistance of the closed switch. PA1 R(.nu..sub.in) is equal to the voltage level dependent on resistance of the closed switch.
The output signal can be filtered to reduce the power level at the undesired frequencies. For instance, if the input signal is centered on a carrier at 240 megahertz (MHz) and the sampling circuit is clocked at 60 MHz, a replica of the modulated input signal appears at baseband, 60 MHz, 120 MHz, 180 MHz, as well as at several higher frequencies. The replicas above the baseband frequency can be filtered such that only the baseband replica is preserved.
The on resistance of the switch 20 is not ideal and, therefore, the switch 24 exhibits ohmic resistance even when the switch 20 is closed. FIG. 2 is a schematic diagram showing an equivalent circuit when the switch 20 is closed. A resistor 26 represents the on resistance of the switch 20. Due to the resistive nature of the closed switch, the output signal is related to the input signal in accordance with Equation 2, below. ##EQU1##
where:
It is evident from examining Equation 2 that the switched capacitor sampling circuit acts as a low pass filter.
In reality, the resistive value of the switch 20 is not constant and instead is a function of the voltage level of the input signal. FIG. 3 is an x/y graph showing the resistive value of an exemplary single nMOSFET switch as a function of the voltage level of the input signal. In FIG. 3, the horizontal axis represents the input signal voltage level in volts. The vertical axis represents the ohmic resistance of the switch on a logarithmic scale in Ohms (.SIGMA.). As shown in FIG. 3, the on resistance of a FET is a strong function of the voltage level of the input signal which is applied to it.
Taking into consideration the curve shown in FIG. 3, Equation 3 more accurately reflects the effect of the on resistance of the switch 20. ##EQU2##
where:
By examining Equation 3, one can see that not only does the switch act as a low pass filter but, in addition, the response of the low pass filter is a function of the voltage level of the input signal. For this reason, the switch is nonlinear and tends to create extremely high levels of distortion to the output signal.
FIG. 4 is a schematic diagram showing a parallel nMOSFET and pMOSFET (metal oxide semi-conductor field effect transistor) switch 24. The parallel switch 24 conducts signals so long as the voltage range of the input signal remains within the power supply voltages used to bias it. The parallel switch 24 exhibits substantially less variance in on resistance as a function of input signal level and, therefore, provides a more linear response.
FIG. 5 is an x/y graph showing the resistive value of a prior art parallel switch as a function of the voltage level of the input signal. In FIG. 5, the horizontal axis represents the input signal voltage level in volts. The vertical axis represents the ohmic resistance of the parallel switch in Ohms (.SIGMA.). Notice that between 1.0 to 1.4 Volts (V) the resistance of the switch varies by about 2.5 (i.e. R(.nu..sub.in =1)*2.5=R(.nu..sub.in =1.4). Such high levels of variance of on resistance as a function input voltage can cause significant distortion in the sampling process.
The frequency response of the on-resistance of prior art parallel switches is also dependent on the input voltage level. FIG. 6 is an x/y graph showing the frequency response of a prior art parallel switch. The solid curve 28 represents the frequency response of the parallel switch at an input voltage level of 1.4V. The dotted curve 30 represents the frequency response of the parallel switch at an input voltage level of 1.0V. FIG. 7 is an x/y graph showing the phase response of a prior art parallel switch. The solid curve 32 represents the phase response of the parallel switch at an input voltage level of 1.4V. The dotted curve 34 represents the phase response of the parallel switch at an input voltage level of 1.0V. The divergence of the high frequency characteristics as a function of the input signal contributes additional nonlinearities to the performance of the switch and tends to more greatly distort the output signal.
When a switch with such non-linear properties is used to subsample a high frequency RF signal, the resultant samples are distorted. Therefore, the resultant samples do not accurately reflect the actual characteristics of the RF signal. As the distorted samples are subject to further processing within the receiver, the distortion produces errors. The errors can be so significant that using the switches at high frequencies is not practical and more expensive, larger and power-hungry down-conversion methods must be employed.
For these reasons, there is a need in the industry to develop a switch which exhibits a more linear response.